This invention relates generally to improved unitized, in-line electron guns for color cathode ray tubes and their manufacture and more specifically to an improved method of mounting of the convergence cup.
Unitized, in-line electron guns generate three coplanar electron beams developed by the thermionic emission of heated cathodes arranged in-line. The resulting beams are formed and focused by a tandem succession of electrodes spaced along the central axis of the gun. The electrodes cause the beams to converge at multiple phosphor groups located on the faceplate of the color cathode ray tube. The prime objective in the design of such guns is to provide small spot size and enhanced resolution. To accomplish this objective, the electron gun electrodes and their field forming surfaces should be accurately spaced, the opposing faces of the electrodes should be parallel, and the beam passageways that extend from the point of beam origin at the cathodes in the base area of the gun and through to the convergence cup at the opposite end, should be coaxially aligned.
This invention is concerned with mounting and proper alignment of the convergence cup which exerts the final influence on the three beams in their respective paths of travel to a point of common convergence on the viewing screen. Contact springs are also mounted on the convergence cup; these center the electron gun within the neck of the cathode ray tube. Also, the contact springs carry the high voltage potential of the conductive coating on the inner surface of the tube neck to the convergence cup. The potential is then transferred to the electrodes that comprise the electron-optical lens of the gun.
The precision with which the convergence cup is aligned with respect to the main gun assembly is a particularly critical factor in the performance of unitized, in-line guns. In such guns, the three beams must be coplanar; that is, they must have their origin in a common cathodic plane, and they must remain in the same plane in their passage throughout the gun. If the convergence cup is mounted in angular misalignment, the "landing line" of the three beams will not lie exactly in the horizontal meridional plane at the viewing screen, with the consdequence that misregistration of beams-to-phosphor-elements on the screen will be severe. Also, any translational misalignment of the convergence cup in a radial direction can exert an equally deleterious effect on gun performance in that the beam passageways will no longer be coaxial. In view of the fact that there are no supplementary means for correcting such types of convergence cup misalignment such as adjustable magnets or pole pieces located externally or internally, the need for a method of precision mounting of the convergence cup at the time of manufacture is mandatory.
The standard method of electron gun assembly is to mount the beam focusing electrodes and the convergence cup sequentially on two parallel mandrels of identical diameters, one round in cross-section and the other diamond-shaped, and so spaced and fixtured as to be concentric with the preferred paths of travel of the two outer electron beams. The electrodes are stacked on the two mandrels along with suitable spacers, with the two mandrels passing through the two outer beam-passing apertures of each electrode. This system of assembly, which is a standard manufacturing procedure, ensures the coaxiality of the three beam passageways, and the proper spacing and parallelism of the components so assembled. The assembly is then bonded into a coherent, mechanically sound structure by a beading process in which two or more glass beads are heat-softened and pressed upon the holding claws of the gun electrodes. Upon cooling of the beads, all parts are permanently fixed in proper relationship one to the other.
This invention finds useful application in connection with a unitized, in-line gun structure and its manufacture. The afore-described method of gun component assembly is not suitable for mounting the convergence cup of the gun described in this disclosure because of the presence of two shunt magnets which encircle the two outer beam apertures where the beams emerge from the base of the convergence cup. To exert their beneficial effect on the two outer beams, it is necessary that the inner diameter of these circular shunt magnets be smaller than the diameter of the outer beam apertures in the gun electrodes that precede the convergence cup. As a result, it is not feasible to use the standard mandrelling process as described.
U.S. Pat. No. 3,873,879 (Hughes) describes a unitized in-line electron gun having a "shield cup" whose configuration and function is similar to that of the convergence cup described in the present invention. Hughes does not address the method of properly registering the shield cup with the other gun electrodes during manufacture to prevent misalignment. The method of shield cup mounting appears to be by welding the cup directly to a flange protruding from the second accelerating and focus electrode.
Dogget et al in U.S. Pat. No. 3,614,502 recite a method for the self-registration of a pole-piece type convergence cage used in a non-unitized, in-line gun of barrel-type construction. Dogget et al teach a self-registering attachment system of the convergence cage. To make this attachment possible, annular flanges are welded to the accelerating grid electrode of each gun, and the center flange has therein two cut-outs for registration with two holes located in the base of the convergence cage. For reasons which will become clear only after the following description of the unique gun with which this invention is advantageously associated and its assembly problems, the Dogget et al disclosure provides no answer to the above-described unitized in-line gun assembly problems.